1. Field of the Invention
The present invention relates to a method for preparing polyurethane, and more particularly to a method for preparing polyurethane having excellent resistance to thermal aging, weather proofing and non-discoloring properties comprising using 3,9-bis(2-hydrazidoethyl)-2,4,8,10-tetraoxaspiro-[5,5]-undecane as a chain extender.
2. Description of the Prior Art
Polyurethane is usually prepared by reacting a prepolymer having terminal isocyanate groups with a chain extender such as diamines. However, the use of diamines such as methylene-bis-ortho chloroaniline has recently been strictly restricted owing to their poisonous property. Therefore, the demand for a novel chain extender having non-poisonous property is increasing. On the other hand, the reactivity of diamine with an isocyanate compound is generally so high that the pot life can not be advantageously controlled and the gelatinization often occurs and the discolored polymer is formed under some reaction condition.
To overcome the defect described above, aliphatic or alicyclic diisocyanate have been often used instead of aromatic diisocyanate. However, the reactivity of the aliphatic or alicyclic diisocyanate is inferior to that of aromatic diisocyanate so that a high reaction temperature or a large amount of the chain extender are required. Moreover, the physical properties, particularly hardness of the resulting polymers are also unsatisfactory.
The present inventors have found 3,9-bis(2-hydrazidoethyl)-2,4,8,10-tetraoxaspiro-[5,5]-undecane to be a novel chain extender, giving a polyurethane of excellent properties.